Regulation of the levels of pyridoxal phosphate (PLP) may play an important role in determining the metabolic capabilities of cells and tissues in that a number of enzymes involved in the intermediary metabolism of amino acids require PLP as a coenzyme. Such enzymes are important in the qualitative and quantitative disposition of amino acids and include transaminases, decarboxylases and enzymes which effect reactions of amino acid side chains. In addition, roles for PLP or PLP-dependent enzymes have been proven or suggested in blood coagulation, regulation of enzyme degradation, lymphoid and immune system function, and in regulating the properties of steroid-receptor complexes. Thus, three enzymes that are involved in the regulation of the amounts of PLP in cells are extremely important. These enzymes are: PLP phosphatase, pyridoxal kinase, and pyridoxine phosphate oxidase. The activities of these enzymes are to be measured under a variety of conditions so as to understand their roles in response to nutritional status, in rapidly growing tissues, such as hepatomas and regenerating liver, and with reference to inter-organ relationships of PLP metabolism.